Security hardening, code quality, and ESLint setup

- Remove production DB dump (db_backup_*.sql) and update.log from git tracking
- Add DB backups to .gitignore
- Replace all console.log/console.error with structured logger module
- Translate Dutch error messages to English (link-discord.ts)
- Remove dead code blocks (register-form.tsx false && pattern)
- Add ESLint flat config with TypeScript, React, Next.js, jsx-a11y, and security plugins
- Add Prettier config
- Add eslint-plugin-security for security-aware linting
- Fix all 119+ ESLint warnings across the codebase:
  - Resolve security/detect-object-injection with safe access patterns
  - Resolve security/detect-non-literal-fs-filename with path traversal validation
  - Replace <img> with next/image <Image> component
  - Remove unused variables and imports
  - Replace non-null assertions with proper type guards
  - Replace <a> with <Link> for internal navigation
  - Use next/script Script component for external scripts
- Fix setState-in-useEffect anti-patterns (navbar-color-picker, logo-generator, theme-switcher)
- Add lint and format scripts to package.json

All checks: typecheck ✓, tests 58/58 ✓, lint 0 errors 0 warnings ✓

src/lib/services/rcon.ts

@@ -1,6 +1,7 @@
 import net from "node:net";
 import { CurrencyType } from "@/lib/services/currency";
 import { env } from "@/env";
+import { logger } from "@/lib/logger";
 
 export interface RconPayload {
   key: string;
@@ -47,7 +48,7 @@ export function tcpTransport(opts: {
         await new Promise((r) => setTimeout(r, 200 * 2 ** attempt));
       }
     }
-    console.error("[RCON] delivery failed after retries:", payload.key);
+    logger.error("RCON delivery failed after retries", { module: "rcon", key: payload.key });
     return false;
   };
 }